Pressure-sensitive adhesives which are used for label applications do not in general remove cleanly from nonpaper substrates. Some residue of the adhesive is left on the substrate thus making its cleaning less effective or more involved. There is a growing need in the market today for environmentally or user friendly pressure-sensitive adhesive labels which can be cleanly removed from the substrate during domestic or industrial cleaning processes. This is true in the case of price marking and product identification labels used on metals, polymers (plastics), glass, wood and painted or water resistant coated substrates.
Attention in this regard has focused on water dispersible, water soluble, repulpable and wet stick adhesives. Most of the base polymers for such adhesives are not inherently pressure-sensitive adhesives. Being nontacky they require the addition of high levels of plasticizers and/or tackifiers. Carboxyl content of the base polymer is also high, exceeding 10% by weight of the polymer, such carboxyl groups are incorporated into the polymer during polymerization. When the base polymer used for a removable adhesive is pressure-sensitive, it is usually compounded with at least 10% by weight of a vinyl carboxylic acid polymer either in the acid or salt form and various amounts of plasticizers and/or tackifiers.
U.S. Pat. No. 4,033,918 to Hauber discloses a water removable pressure-sensitive adhesive composition which comprises based on 100 parts weight of the total solids, of from 25 to 35 parts by weight of a crosslinked acrylic acid ester copolymer which contains 10 to 20% by weight of unsaturated carboxylic acid, vinyl acetate, 2-ethyl hexyl acrylate, acrylonitrile, diethyl maleate and mixtures thereof, from 10 to 20 parts by weight of an uncrosslinked acrylic acid ester copolymer, from 10 to 20 parts by weight of polyvinylmethyl ether, from 10 to 20 parts by weight of an alkali metal and/or ammonium salt of polyacrylic acid, polymethacrylic acid or a copolymer of acrylic and methacrylic acids; 10 to 20 parts by weight of monomeric plasticizer such as a polyvinyl methyl ether or phthalate ester and 10 to 15 parts by weight of an emulsifying agent such as ethoxylated phenols.
U.S. Pat. Nos. 3,865,770, 4,413,080 and 4,569,960 to Blake disclose tapes providing water dispersible/water soluble pressure-sensitive adhesives for use as a repulpable splicing tape. The adhesives described are solvent polymerized alkyl acrylate and acrylic acid polymers, neutralized with alkanolamines, or potassium hydroxide used in combination with sodium hydroxide and/or lithium hydroxide. The polymer is further blended with acidic rosin or rosin derivatives and plasticized with hydrophilic polyoxyethylene compounds. Adhesive compositions comprise a polymer containing 60-85% alkyl acrylate and 10 to 40% vinyl carboxylic acid. The formulated adhesives have 35 to 150 parts by weight of acidic rosin, and/or 50 to 250 parts by weight of surfactant (ethoxylated alkyl phenols and/or complex acidic esters of phosphoric acid). High levels of plasticizer and high levels of rosin acid pose aging problems due to separation or migration thereby decreasing the adhesive properties. The adhesive tapes are sold in closed containers to preserve adhesive properties which are rapidly lost on exposure to atmospheric humidity.
U.S. Pat. No. 4,492,724 to Albright, et al. describes a humidity resistant wet-stick pressure-sensitive adhesive made up of three components. Two are nonpressure-sensitive polymeric components containing high levels of polymerizable acids and the third component consisting of an ethoxylated alkyl arylphenol which serves as a plasticizer/tackifier to make the composition a pressure-sensitive adhesive. The first component is a copolymer of vinyl acetate, alkyl acrylate and acrylic acid in which the acrylic acid is present in an amount of from 20 to 55% by weight, the second component is an acrylic acid polymer which is present from 8 to 35% by weight and the plasticizer is present in an amount of from 20 to 65% by weight.
U.S. Pat. No. 4,413,082 to Gleichenhagen, et al. teaches a water soluble pressure-sensitive adhesive composition for repulpable splicing tape comprising from 30 to 80 parts of a copolymer of 70 to 88% by weight of butyl acrylate of which 5 to 25% can be a mixture of vinyl acetate and fumaric acid dialkyl ester and 12 to 30% by weight vinyl carboxylic acid blended with from 20 to 70 parts by weight of a water soluble plasticizer which is ethoxylated alkyl phenol, ethoxylated alkyl mono or diamine and from 2 to 30 parts by weight of an acidic rosin. The adhesive is water soluble and contains more than 10% of vinyl carboxylic acid in the polymer.
U.S. Pat. No. 4,418,120 to Kealy, et al. relates to a tackified crosslinked acrylic adhesive which display improved adhesion to low energy substrates. The adhesive is based on a solution polymer of isooctyl acrylate and acrylic acid containing from 3 to 7% by weight acrylic acid, 100 parts of which is blended with 20 to 50 parts of a tackifying rosin ester having an acid number less than 20, additionally the adhesive contains certain antioxidants and a crosslinking agent, is coated and dried at 70.degree. C. to allow for crosslinking. The composition is required to be free of surfactants to achieve the designated performance.
U.S. Pat. No. 4,726,982 to Traynor, et al. relates to a tackified acrylic pressure-sensitive adhesive for application to high solids automotive paints. The adhesive is also a solution polymer of acrylic acid ester of nontertiary alcohol containing from 4 to 14 carbon atoms, which polymers contain from 10 to 40 parts of N-vinyl lactam and is tackified with 5 to 50 parts, based on the weight of the polymer, of tackifying resins such as isobornyl methacrylate, rosin ester, and/or mixed aliphatic/aromatic polymeric tackifier resins. A suitable combination of the N-vinyl lactam and the tackifier is needed to provide high adhesion to automotive paint. Compositions with less than 10% N-vinyl lactam do not provide the high adhesion. The tackifying resins are either esters or hydrocarbon resins with no carboxyl functionality.